This invention relates in general to a control method for enhancing stability of a vehicle by adjusting brake pressure following deflation of a tire to a lower pressure.
In the event of a rapid tire deflation or blowout in a motor vehicle, the deflated tire is unable to support the same forces and moments as the inflated tire on the opposite side of the same axle.
At least two factors are important in this respect. First, the longitudinal force between the road and the tire created by a given braking torque can be lower for a deflated tire than an inflated tire because a delay in transmitting brake torque to the road is increased at the deflated tire. Therefore, there is a brief period during which the inflated tire is creating a larger force. Additionally, the maximum coefficient of friction between the tire and road may be decreased in a deflated tire, thereby reducing the maximum longitudinal force that can be created between the tire and road. The result, in either case, is a difference in magnitude of longitudinal forces at the left and right wheels of a given axle, which induces a yaw moment on the vehicle and tends to cause the vehicle to yaw or spin. The driver can counteract this yaw with steering input if his reaction is quick enough, if his reaction is correct, and if adequate lateral force capability exists at the tires.
Second, the lateral force that can be supported by a deflated tire is greatly reduced when compared to an inflated tire. Therefore, if the vehicle is turning, there may be inadequate lateral force to maintain the desired turning radius. With a front tire, the result is understeer, which induces the vehicle to plow out of the turn, while with a rear tire, the result is oversteer, which induces vehicle yawing or spinning about a vertical axis. Application of braking forces to a tire reduces its lateral force support capability, so applying braking force to a deflated tire further reduces its capability.
These two factors can work singly or in combination such that a vehicle, initially going in a straight line, may be caused to spin by the first factor. The second factor causes inadequate lateral force capability to control the spin which then may increase out of control.
Driver reaction to a blowout is often a rapid release of the throttle and application of the brakes. The brakes generate roughly equal pressures at both the inflated and deflated tire on the affected axle. These equal pressures may result in unequal forces and moments on the vehicle as described above. Therefore, it would be desirable to provide a system and method for enhancing stability of a vehicle with a deflated tire.